First off, I wouldn't worry about it. The 8400GS is not powerful enough to use up the x16 bandwidth in the first place. Even high end cards like a GTX 470 will run pretty decent on an x8/x8 setting when setup in an SLI pair.
Secondly, as for why it's running in x8. This could be an issue of drivers (are you up to date?). It could also be an issue of BIOS settings on the motherboard as well. Which Motherboard do you use? Depending on your board, and the PCI-E slot you're in, it could only be an x8 slot to begin with. Or it might require a BIOS update to work properly as an x16 with this card, or something like that.
Regardless, from a performance standpoint, the x8 setting is more than enough for your 8400GS. You're not missing anything.